Deimos Dust CO/LOX potato gun thruster and Orbital CO2 mining

[IsaacKuo]

Been a while since I posted an idea here. Anyway, I have come to realize that Deimos regolith dust could be used to power LMO atmospheric scooping. No solar electric thruster is required. Instead, the propulsion system is a variable Isp "potato gun" using CO/LOX fuel to shoot slugs of Deimos regolith dust out of a barrel.

The potato gun thruster packs slugs of Deimos dust at one end of a gun barrel, and uses a bit of CO/O2 mix at the breech to accelerate it out the barrel for thrust. By varying the slug and/or CO/O2 mass, specific impulse can be tuned between 50s (35:1 ratio) and 210s (1:1 ratio).
 
For maximum efficiency, it is best to vary specific impulse with delta-v expended so far plus a constant. For simplicity, though, I will describe with constant specific impulse. Just know that the CO/LOX consumption would be significantly lowered by taking advantage of variable specific impulse.

1) The mining drone loads up with about 3 tons of regolith dust at Deimos. This might be gathered by an statically charged coil torus forced through a scraper.

2) At a 35:1 dust to COLOX ratio, the mining drone thrusts 560m/s to Mars escape. This consumes around 2 tons of regolith dust and 0.06 tons of COLOX. This leaves about 1 ton of spacecraft mass (mostly regolith, but also the spacecraft's dry mass).

3) The escape trajectory is chosen so it returns to Mars on an atmosphere grazing trajectory. This means it encounters the upper atmosphere at about 1.414 times orbital velocity. Thus, there's enough momentum to gather about .4 tons of CO2/argon.

4) Before each pass, the regolith is radiatively cooled to well below dry ice temperature. This will likely also be below argon's melting point.

5) During each pass, scooped atmosphere is frozen into dry ice and liquid argon. Nitrogen is vented.

6) After each pass, solar heating is used to transfer CO2 and argon into separate storage tanks (the reglith tank is heated; the other tanks are shaded). This prevents dry ice buildup from reducing the useful heat transfer area of the regolith.

7) After about .4 tons of dry ice/argon are gathered, the mining drone is in a near circular orbit. It travels to a refinery in LMO.

8) At the LMO refinery, it transfers the ice/argon in exchange for about 0.13 tons of COLOX. Some regolith may optionally be transferred.

9) The potato gun thruster is used to return to Deimos. Assuming a fixed 210s specific impulse is used, this consumes around 0.07 tons of regolith (out of 1 ton available) and 0.07 tons of CO+LOX. A total of about 0.13 tons of CO+LOX is needed--much less than the .4 tons of dry ice delivered. Note that CO+LOX consumption can be significantly reduced by using some more regolith, and the amount of regolith used in this example is very small.

OTOH, I'm neglecting the dry mass of the mining drone itself, which would add about its own mass in terms of the CO+LOX required.

The bottom line, though, is that the mining drone can deliver a decent chunk of dry ice to LMO with each cycle. Most of the energy comes from the orbital energy of Deimos, which means solar panel resources can be devoted almost entirely to refining CO2 into CO+LOX. The system does not use an electric thruster to bring the scooped CO2 up to orbital speeds. It's essentially a fancy way to exchange regolith high in the gravity well for dry ice low in the gravity well.

Additionally, cargo drones could bring CO+LOX to Earth orbit. It only costs around 2.3km/s to get from LMO to Earth. Assuming 210s, that implies a mass ratio of around 3:1, or a 1:1:1 ratio of regolith:CO+LOX:payload. The delivered dry ice/argon/regolith could then be used for various things in Earth orbit, including being used to fuel hypersonic skyhitch.

[qraal]

Why not just dangle a skyhook off Phobos to scoop at much lower speeds than an orbital pass?

[IsaacKuo]

Why not just dangle a skyhook off Phobos to scoop at much lower speeds than an orbital pass?

Uh...because Phobos is at an altitude of 6000km? A 6000km long tether is far beyond the state of the art and would represent a massive initial investment even if we could build it.

The proposed system uses two conventionally sized spacecraft (the mining drone and the solar powered refinery), with pretty conventional hardware. A new thruster must be developed, but it's fundamentally a potato gun.

[Hobbes-22]

So you'd be using this in Mars orbit? This seems a recipe for getting to Kessler syndrome levels of orbital debris as quickly as possible.

[IsaacKuo]

So you'd be using this in Mars orbit? This seems a recipe for getting to Kessler syndrome levels of orbital debris as quickly as possible.

The thruster is only used at a very high altitude (Deimos orbit) and at a very low orbit in a direction which gives the projectiles very suborbital velocity (they almost immediately enter Mars's atmosphere). The high orbit dust debris would be spread over an extremely large volume, and since it's Deimos crossing it may not stick around for all that long anyway.

Note that the orbital velocities around Deimos orbit are only on the order of a km/s; since they're all going prograde collision velocities would be pretty low.

[Robotbeat]

You can use dust and gas directly as propellant. Use a resistojet to heat and accelerate the gas, then inject the dust to increase mass flow.

Something like this is used for "cold spray" welding.

[IsaacKuo]

You can use dust and gas directly as propellant. Use a resistojet to heat and accelerate the gas, then inject the dust to increase mass flow.

Something like this is used for "cold spray" welding.

Yes, but this requires electrical power. Unless you have a lot of electrical power, you're going to have low acceleration and lose the advantage of the Oberth effect. (Spiraling outward vs impulsive Hohmann transfer.)

Also, this system leaves the large solar arrays in LMO rather than dragging them around from orbit to orbit. This minimizes the dry mass of the mining drone.

[Robotbeat]

This propulsion method would be low Isp, which means even with electric, it'd be decently high thrust. Easily high enough to get you off of the moons. And if you thrust at perigee, you still get Oberth effect just fine.

If I get 1Newton of thrust at 5000s Isp, the at 50s Isp and the same power, you get 100x that, or 100N of thrust. Comparable to chemical thrusters, and plenty good enough for the concept outlined here.

[Lar]

The exact propulsion isn't quite as important (either one sketched out would work..) as the idea of atmo mining that exchanges mass to put useful mass where you want it. Nifty.

[IsaacKuo]

This propulsion method would be low Isp, which means even with electric, it'd be decently high thrust. Easily high enough to get you off of the moons. And if you thrust at perigee, you still get Oberth effect just fine.

If I get 1Newton of thrust at 5000s Isp, the at 50s Isp and the same power, you get 100x that, or 100N of thrust. Comparable to chemical thrusters, and plenty good enough for the concept outlined here.

That requires on the order of 10 kilowatts, implying something like 100 square meters of solar panels. Heavy and big, and they need to be folded up during aeroscooping.

No, electric propulsion trying to operate for minutes isn't going to compete with a refinery operating over days.

[Hanelyp]

Have you accounted for the temperature equivalent of velocity on the gas processed in the scoop pass?  The gas is going to be heated a LOT by the ram scoop.

An electric sling shot seems a lot less trouble than the ram scoop.

[Robotbeat]

This propulsion method would be low Isp, which means even with electric, it'd be decently high thrust. Easily high enough to get you off of the moons. And if you thrust at perigee, you still get Oberth effect just fine.

If I get 1Newton of thrust at 5000s Isp, the at 50s Isp and the same power, you get 100x that, or 100N of thrust. Comparable to chemical thrusters, and plenty good enough for the concept outlined here.

That requires on the order of 10 kilowatts, implying something like 100 square meters of solar panels. Heavy and big, and they need to be folded up during aeroscooping.

No, electric propulsion trying to operate for minutes isn't going to compete with a refinery operating over days.

If it's just minutes and the delta-v isn't too high, batteries are competitive.

[IsaacKuo]

Have you accounted for the temperature equivalent of velocity on the gas processed in the scoop pass?  The gas is going to be heated a LOT by the ram scoop.

An electric sling shot seems a lot less trouble than the ram scoop.

I have no idea what you mean by using an electric sling shot. The ram scoop needs two things to operate successfully - one which I explicitly mentioned, and one which I did not.

I explicitly mentioned the cooling mechanism. The regolith is cooled to well below dry ice temperatures. This converts the incoming hot CO2 into dry ice. There's only so much it can convert in a single pass.

What I did not mention is the throat pump, required to boost internal pressure above ambient. This is a centrifugal pump at the throat directly connected to a flywheel. This also has a limit to how much it can pump in a single pass before it loses too much velocity.

[IsaacKuo]

This propulsion method would be low Isp, which means even with electric, it'd be decently high thrust. Easily high enough to get you off of the moons. And if you thrust at perigee, you still get Oberth effect just fine.

If I get 1Newton of thrust at 5000s Isp, the at 50s Isp and the same power, you get 100x that, or 100N of thrust. Comparable to chemical thrusters, and plenty good enough for the concept outlined here.

That requires on the order of 10 kilowatts, implying something like 100 square meters of solar panels. Heavy and big, and they need to be folded up during aeroscooping.

No, electric propulsion trying to operate for minutes isn't going to compete with a refinery operating over days.

If it's just minutes and the delta-v isn't too high, batteries are competitive.

No way. The specific energy stored in batteries is far lower than CO/LOX. This battery mass would have to be hauled all the way back from LMO to Deimos, which kills the entire system's viability.

[IsaacKuo]

The exact propulsion isn't quite as important (either one sketched out would work..) as the idea of atmo mining that exchanges mass to put useful mass where you want it. Nifty.

Thanks! Yeah, that's the basic idea. It is pretty sensitive to the amount of mass you need to bring back from LMO to Deimos, though. At first, I thought of this idea using a pure CO/LOX thruster - not augmented by Deimos dust. The numbers for that idea just didn't work. You'd need to spend more CO/LOX than you'd get back without some sort of thrust augmentation.

[Robotbeat]

This propulsion method would be low Isp, which means even with electric, it'd be decently high thrust. Easily high enough to get you off of the moons. And if you thrust at perigee, you still get Oberth effect just fine.

If I get 1Newton of thrust at 5000s Isp, the at 50s Isp and the same power, you get 100x that, or 100N of thrust. Comparable to chemical thrusters, and plenty good enough for the concept outlined here.

That requires on the order of 10 kilowatts, implying something like 100 square meters of solar panels. Heavy and big, and they need to be folded up during aeroscooping.

No, electric propulsion trying to operate for minutes isn't going to compete with a refinery operating over days.

If it's just minutes and the delta-v isn't too high, batteries are competitive.

No way. The specific energy stored in batteries is far lower than CO/LOX. This battery mass would have to be hauled all the way back from LMO to Deimos, which kills the entire system's viability.

The extra mass can be an asset as a momentum buffer if you do multiple passes when collecting CO2. I think it's likely that multiple passes will be required anyway.

[Hanelyp]

Have you accounted for the temperature equivalent of velocity on the gas processed in the scoop pass?  The gas is going to be heated a LOT by the ram scoop.

An electric sling shot seems a lot less trouble than the ram scoop.

I have no idea what you mean by using an electric sling shot.

One option would resemble a baseball pitching machine adapted for higher throwing speed.

What I did not mention is the throat pump, required to boost internal pressure above ambient.

That pump is going to heat the gas even more.

[IsaacKuo]

No way. The specific energy stored in batteries is far lower than CO/LOX. This battery mass would have to be hauled all the way back from LMO to Deimos, which kills the entire system's viability.

The extra mass can be an asset as a momentum buffer if you do multiple passes when collecting CO2. I think it's likely that multiple passes will be required anyway.

That's not the problem. The problem is hauling all that extra mass up from LMO to Deimos. This scheme is sensitive to how much mass that is, compared to how much mass is harvested on the way down. Because CO/LOX has good specific energy, the mass that must be brought up costs less than half of the amount of dry ice harvested. But the specific energy of a battery is an order of magnitude lower, so it just...doesn't work.

Note that I explicitly describe that multiple passes are used when atmospheric scooping. Each pass lowers the apoapsis a bit. But only a little bit can be done in each pass, due to the limited heat capacity of the cold regolith. The incoming CO2 has a very high specific energy because it's encountering the mining drone at 3.5 to 5 km/s.

[IsaacKuo]

An electric sling shot seems a lot less trouble than the ram scoop.

I have no idea what you mean by using an electric sling shot.

One option would resemble a baseball pitching machine adapted for higher throwing speed.

I don't see how that can be used to scoop up CO2 from Mars's upper atmosphere. That's what the ram scoop is used for.

The goal is to harvest CO2.

What I did not mention is the throat pump, required to boost internal pressure above ambient.

That pump is going to heat the gas even more.

This can't be helped. Without a throat pump, pressure will build up within the scoop until most of the atmosphere spills around the spacecraft rather than funneling through the throat. Think about what happens when you push a funnel backwards through a fluid. A tiny stream of fluid will squirt through the tip but most of the fluid will just get pushed aside (with a lot of drag).

But with a throat pump and a sufficiently large throat, you can slurp up fluid fast enough to ensure all of the fluid reaching the scoop will get sucked in.

I don't know for sure...maybe if the regolith freezes the incoming hot CO2 fast enough, the volume reduction effect of freezing CO2 will provide a sufficient "suction" effect. But everything I've read on atmospheric scooping uses a throat pump.

[IsaacKuo]

Thinking more about it, it may make more sense to put the refinery in an atmosphere grazing elliptical orbit with apoapsis at Deimos height. Simple momentum absorbing to "aerobrake" down to LMO is less efficient than using the thruster to spit regolith rearward at periapsis.

This puts the aeroscoop on the refinery rather than the mining drone. The mining drone has no aeroscoop, only a potato gun thruster and CO/LOX tanks. Now it only takes 670m/s delta-v to go back to Deimos from the refinery. (It takes 560m/s to go to from Deimos to C3, and then aeroscoop down to the refinery.)

There's a window every 7 orbits (90 hours) to go from the refinery back to Deimos (3 orbits in 90 hours).

In this case, simple momentum only gives you a pathetic 6% conversion of mass at C3 to dry ice. The target periapsis speed of 4.7km/s is not much lower than the initial periapsis speed of 5km/s. The real deal comes from using the potato gun thruster at periapsis to spit regolith rearward.

Ideally, some sort of aeroramjet propulsion system would be used, but that would be harder to develop than a simple potato gun. An aeroramjet channels some of the hot incoming air to a nozzle with dust injection. This avoids wasting energy cooling the hot air to dry ice, and using electrical energy to convert it into CO/LOX (or charging a battery to power a resistojet). But like I said, it's challenging to develop this thruster.

Anyway, placing the refinery at this elliptical orbit reduces the rate at which CO2 is harvested, but this CO2 is gathered at a point much higher in the gravity well. This means that it's a heck of a lot closer to delivery to Earth, in terms of delta-v.